Piezoelectric/electrostrictive film element

ABSTRACT

A method of producing a piezoelectric/electrostrictive film element is disclosed. The film element includes a zirconia substrate having a window which is closed by a diaphragm portion, and a piezoelectric or electrostrictive unit formed on the substrate and including a piezoelectric or electrostrictive layer between upper and lower electrodes. The method comprises the steps of: (a) preparing the substrate which has been sintered and in which at least the diaphragm portion contains alumina in an amount of 1.1-5.0 parts by weight; (b) forming, by a film-forming process, the lower electrode on the diaphragm portion, and the piezoelectric/electrostrictive layer on the lower electrode by using a piezoelectric/electrostrictive material which contains magnesia or a component which gives magnesia, in an independent form or in a compound form; and (c) firing the piezoelectric/electrostrictive layer, so as to deposit particles consisting principally of a compound of alumina and magnesia at least on an interface between the diaphragm portion and the lower electrode, the interface being located right under the piezoelectric/electrostrictive layer.

This is a divisional application of U.S. Ser. No. 08/716,876, filed Sep.20, 1996, now U.S. Pat. No. 5,853,514 the entirety of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a uni-morph, bi-morph or other types ofpiezoelectric and/or electrostrictive film elements which generate ordetect displacement or force in the form of bending, deflection orflexure, and which can be used for actuators, filters, display devices,transformers, microphones, sounding bodies (such as loud speakers),various vibrators, resonators, or oscillators, discriminators, gyros,sensors and other components and devices. The present invention is alsoconcerned with a method of producing such piezoelectric and/orelectrostrictive film elements. The term “element” used herein means anelement which is capable of transducing or converting an electric energyinto a mechanical energy, i.e., mechanical displacement, strain orvibrations, or converting a mechanical energy into an electric energy.

2. Discussion of the Related Art

In recent years, in the fields of optics and precision positioning ormachining operations, there has been an increasing demand for an elementwhose displacement is controlled for adjusting or controlling an opticalpath length or a position of a member or component of a device, on theorder of fractions of a micron (μm), and a detecting element adapted todetect infinitesimal displacement of a subject as an electric change. Tomeet the demand, there have been developed piezoelectric and/orelectrostrictive film elements (hereinafter referred to as “P/E filmelements”) used for actuators or sensors, which elements comprise apiezoelectric material such as a ferroelectric material, and utilize thereverse or converse piezoelectric effect to produce a mechanicaldisplacement upon application of an electric field to the piezoelectricmaterial, or utilize the piezoelectric effect to produce an electricfield upon application of a pressure or mechanical stress. Among theseelements, a conventional uni-morph type P/E film element has beenfavorably used for a loudspeaker, for example.

There have been proposed ceramic P/E film elements used for variouspurposes, as disclosed in JP-A-3-128681 (i.e., in the co-pending U.S.patent applications Ser. Nos. 07/550,977, 07/860,128, 08/102,960,08/384,469, 08/392,083 and 08/452,092) and in JP-A-5-49270 (i.e., inU.S. Pat. No. 5,210,455 and U.S. patent application Ser. No.08/013,046), which were filed by the assignee of the present invention.One example of the disclosed elements has a ceramic substrate which hasat least one window, and is formed integrally with a thin diaphragmwhich closes the window or windows so as to provide at least onethin-walled diaphragm portion or vibratile portion. On an outer surfaceof each diaphragm portion of the ceramic substrate, there is formed apiezoelectric/electrostrictive unit (hereinafter referred to as “P/Eunit”) which is an integral laminar structure consisting of a lowerelectrode, a piezoelectric/electrostrictive layer (hereinafter referredto as “P/E layer”) and an upper electrode. The P/E unit is formed by asuitable film-forming method or process on the corresponding diaphragmportion of the ceramic substrate. The thus formed P/E film element isrelatively small-sized, inexpensive, and can be used as anelectromechanical transducer having high reliability. Further, thiselement has a high operating response, and provides a relatively largeamount of displacement by application of a low voltage, with arelatively large magnitude of force generated. Thus, the above-describedelement is favorably used as a member for an actuator, filter, displaydevice, sensor or other components or devices.

To produce the P/E film element as described above, the lower electrode,P/E layer and upper electrode of each P/E unit are laminated in thisorder by a suitable film-forming method, on the diaphragm portion of theceramic substrate which has been sintered. The thus formed P/E unit issubjected to heat treatment (firing or sintering) as needed, so that theP/E unit is formed integrally on the diaphragm portion. A further studyof the inventors of the present invention revealed that thepiezoelectric/electrostrictive characteristics of the film element isdeteriorated due to the heat treatment (firing or sintering) effectedduring the formation of the P/E unit, more specifically, the P/E layer.

That is, the P/E layer suffers from stresses due to firing shrinkage ofthe P/E layer or P/E unit which is in contact with the diaphragm portionof the ceramic substrate, during the heat treatment (firing orsintering) of the P/E layer. As a result, the P/E layer may not besufficiently sintered due to the stresses, and still suffers fromstresses remaining therein after firing. In this case, the P/E filmelement cannot exhibit its inherent piezoelectric/electrostrictivecharacteristics.

In order to increase the sinterability and density of the P/E layer soas to improve the piezoelectric/electrostrictive characteristics of thefilm element, the firing temperature of the P/E layer may be increased,or the thickness of the diaphragm portion on which the P/E layer isformed may be reduced. However, these solutions are not yet effective toimprove the sinterability of the P/E layer and to eliminate the problemof stresses remaining in the P/E layer after firing thereof. That is,the stresses remaining in the P/E layer may deteriorate thepiezoelectric/electrostrictive characteristics of the film element. Inparticular, such residual stresses may reduce the amount of displacementof the diaphragm portion upon actuation of the P/E unit. The reductionin the thickness of the diaphragm portion makes it difficult to producethe ceramic substrate, and lowers the resonance frequency of the P/Efilm element.

The P/E film element suffering from such residual stresses is notcapable of providing sufficient bonding strength between the diaphragmportion of the ceramic substrate and the P/E unit (the lower electrode).As a result, the P/E unit may be separated or peeled off from thediaphragm portion of the ceramic substrate during production and use ofthe P/E film element. Accordingly, the operating reliability of theelement is undesirably deteriorated.

SUMMARY OF THE INVENTION

It is therefore a first object of the present invention to provide amethod of producing a piezoelectric/electrostrictive film elementwherein each piezoelectric/electrostrictive unit is formed by afilm-forming method on an outer surface of a thin-walled diaphragmportion of a zirconia substrate, and wherein apiezoelectric/electrostrictive layer of each P/E unit exhibits highsinterability or density without being affected by the diaphragm portionwhile the bonding strength between the diaphragm portion and the P/Eunit is effectively improved, so as to assure high reliability and highelectromechanical conversion efficiency of the film element.

It is a second object of the invention to provide such apiezoelectric/electrostrictive film element using the method.

The above-indicated first object of the invention may be attainedaccording to a first aspect of the invention which provides a method ofproducing a piezoelectric/electrostrictive film element comprising: azirconia substrate having at least one window, and a diaphragm portionformed as an integral part of the zirconia substrate and closing each ofat least one window; and a film-like piezoelectric/electrostrictive unitincluding a lower electrode, a piezoelectric/electrostrictive layer andan upper electrode, which are formed in the order of description on anouter surface of the diaphragm portion by a film-forming process, themethod comprising the steps-of: (a) preparing the zirconia substratewhich has been sintered and in which at least the diaphragm portioncontains alumina in an amount of 1.1-5.0 parts by weight; (b) forming,by a film-forming process, the lower electrode on the outer surface ofthe diaphragm portion, and the piezoelectric/electrostrictive layer onthe lower electrode by using a piezoelectric/electrostrictive materialwhich contains magnesia or a component which gives magnesia, in anindependent form or in a compound form; and (c) firing thepiezoelectric/electrostrictive layer, so as to deposit particlesconsisting principally of a compound of alumina and magnesia at least onan interface between the diaphragm portion and the lower electrode, theinterface being located right under the piezoelectric/electrostrictivelayer.

According to one preferred form of the first aspect of the invention,the particles deposited at least on the interface are particles ofspinel (MgAl₂O₄).

According to another preferred form of the first aspect of theinvention, the piezoelectric/electrostrictive material includes magnesiaas a component which exhibits piezoelectric/electrostrictivecharacteristics.

According to a further preferred form of the first aspect of theinvention, the diaphragm portion of the zirconia substrate has anupwardly convex shape protruding in a direction away from the window.

In the piezoelectric/electrostrictive film element according to thepresent invention, the diaphragm portion of the zirconia substrateincludes a predetermined amount of alumina while thepiezoelectric/electrostrictive material for thepiezoelectric/electrostrictive layer includes, in an independent form orin a compound form, magnesia or the component which gives magnesia.During the firing process of the piezoelectric/electrostrictive layer,magnesia or the component which gives magnesia moves toward theinterface between the diaphragm portion and the lower electrode whilealumina in the diaphragm portion moves toward the surface of thediaphragm portion, so that magnesia or the component which givesmagnesia reacts with alumina, whereby the particles of the compound ofalumina and magnesia, such as spinel, are deposited on the interface.According to this arrangement, the rigidity of the diaphragm portion islowered or the diaphragm portion is likely to be deformed during thefiring of the piezoelectric/electrostrictive layer, to thereby provide asufficiently dense piezoelectric/electrostrictive layer. Further, thepresent arrangement is effective to eliminate or reduce theconventionally experienced problem of the stresses remaining in the P/Elayer, assuring high operating reliability and high electromechanicalconversion efficiency of the piezoelectric/electrostrictive filmelement. The film element obtained according to the present inventiondoes not suffer from reduction in rigidity of the diaphragm portionafter firing the P/E layer, and therefore the resonance frequency of thefilm element is not lowered.

The second object of the present invention may be attained according toa second aspect of the invention which provides apiezoelectric/electrostrictive film element, comprising: a zirconiasubstrate having at least one window, and a diaphragm portion formed asan integral part of the zirconia substrate and closing each of at leastone window; a film-like piezoelectric/electrostrictive unit including alower electrode, a piezoelectric/electrostrictive layer and an upperelectrode, which are formed in the order of description on an outersurface of the diaphragm portion by a film-forming process; and thediaphragm portion and the lower electrode defining an interface rightunder the piezoelectric/electrostrictive layer, particles consistingprincipally of a compound of alumina and magnesia existing at least onthe interface at an occupied area ratio of 5% or greater.

According to one preferred form of the second aspect of the invention,the compound of alumina and magnesia is spinel (MgAl₂O₄). The particlesof the compound such as spinel existing on the interface between thediaphragm portion and the lower electrode provide an anchoring effectfor bonding the diaphragm portion and the lower electrode to each other,to thereby assure enhanced bonding strength or adhesion therebetween.

DETAILED DESCRIPTION OF THE DRAWINGS

The above and optional objects, features and advantages of the presentinvention will be better understood by reading the following detaileddescription of the presently preferred embodiments of the invention,when considered in connection with the accompanying drawings, in which:

FIG. 1 is a cross sectional view showing one example of a basicstructure of a piezoelectric/electrostrictive film element according tothe present invention;

FIG. 2 is a view observed by an electron microscope, showing a surfaceof the diaphragm portion with the piezoelectric/electrostrictive unitbeing removed;

FIG. 3 is a cross sectional view showing another example of thepiezoelectric/electrostrictive film element according to the invention;and

FIG. 4 is an exploded perspective view of thepiezoelectric/electrostrictive film element of FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

There will be hereinafter described one embodiment of apiezoelectric/electrostrictive film element according to the presentinvention, wherein a piezoelectric/electrostrictive unit is formed by afilm-forming method on an outer surface of a diaphragm portion whichcloses a window formed in a zirconia substrate, and wherein particlesconsisting principally or essentially of a compound of alumina andmagnesia are deposited on the interface between the diaphragm portionand the lower electrode, which interface is located right under thepiezoelectric/electrostrictive layer. In this substrate, the zirconiasubstrate has one window.

Referring to FIG. 1 showing one example of apiezoelectric/electrostrictive film element (hereinafter referred to as“P/E film element) according to the present invention, a zirconiasubstrate 2 has an integral structure which consists of a base plate 4having a predetermined thickness, the base plate having a rectangularwindow 6 of a suitable size, and a relatively thin diaphragm plate 8which closes the window 6. The diaphragm plate 8 is superposed on one ofthe opposite major surfaces of the base plate 4 which serves as asupport member. The diaphragm plate 8 has a diaphragm portion 10 whichcorresponds to the window 6 of the base plate 4. On the outer surface ofthe diaphragm portion 10 of the planar zirconia substrate 2, a lowerelectrode film 12, a piezoelectric/electrostrictive layer 14(hereinafter referred to as P/E layer), and an upper electrode 16 arelaminated in this order by a known film-forming method, so as to providea film-like piezoelectric/electrostrictive unit 18 (hereinafter referredto as P/E unit). As known in the art, a suitable voltage is applied tothe lower and upper electrodes 12, 16 through the respective leadportions not shown.

Where the P/E film element constructed as described above is used as anactuator, a voltage is applied between the two electrodes 12, 16 of theP/E unit 18 in a known manner, so that the P/E layer 14 is exposed to anelectric field, and undergoes a mechanical distortion induced by theelectric field. Consequently, the P/E unit 18 causes a flexing, bending,or deflecting displacement or force due to a transverse effect of thedistortion of the P/E layer 14, such that the displacement or force actson the zirconia substrate 2 (diaphragm portion 10) in a directionperpendicular to the plane or major surfaces of the zirconia substrate2.

In producing the P/E film element according to the present invention, asintered zirconia substrate 2 is prepared which includes 1.1-5.0% byweight of alumina at least in the diaphragm portion 10. After the lowerelectrode 12 is formed on the outer surface of the diaphragm portion 10of the thus prepared zirconia substrate 2, the P/E layer 14 is formed onthe lower electrode 12 by using a piezoelectric/electrostrictivematerial which contains magnesia or a component which gives magnesia, inan independent form or in a compound form. The thus formed P/E layer 14is fired, whereby particles consisting principally of a compound ofalumina and magnesia, such as spinel (MgAl₂O₄), are deposited at leaston the interface between the diaphragm portion 10 of the zirconiasubstrate 2 and the lower electrode 12 of the P/E unit 18, whichinterface is located right under the P/E layer 14. According to thisarrangement, the rigidity of the diaphragm portion 10 is lowered duringthe heat treatment (firing) of the P/E layer 14, and the diaphragmportion 10 is likely to be deformed. Thus, this arrangement is effectiveto sufficiently improve the sinterability of the P/E layer 14 and toavoid the problem of the residual stresses due to the firing shrinkageof the P/E layer 14.

In the P/E film element according to the present invention, the zirconiasubstrate 2 which carries the P/E unit 18 thereon is favorably made of aknown stabilized zirconia material or a known partially stabilizedzirconia material. Particularly favorably used is a material asdisclosed in JP-A-5-270912 (i.e., in U.S. Pat. No. 5,430,344 and U.S.patent application Ser. No. 08/444,930), which material contains as amajor component zirconia which is partially stabilized by adding acompound(s) such as yttria, and which has a crystal phase that consistsessentially of a tetragonal phase or mixture of at least two kinds ofcubic, tetragonal and monoclinic phases. The crystal grain size of thezirconia substrate 2 is preferably controlled to be not greater than 1μm. The zirconia substrate 2 made of the above-described materialexhibits high mechanical strength and high toughness even with a smallthickness, and is less likely to chemically react with thepiezoelectric/electrostrictive material.

In producing the P/E film element according to the present invention,there is initially prepared the zirconia substrate 2 formed of theabove-described zirconia material, wherein 1.1-5.0% by weight of aluminais included at least in the diaphragm portion 10. The alumina may beincluded in other portions of the zirconia substrate 2 such as the baseplate 4, in addition to the diaphragm portion 10. If the amount ofalumina is excessively large, the diaphragm portion 10 of the zirconiasubstrate 2 may undesirably suffer from cracking or other defects. Inview of this, it is desirable that the amount of alumina be generallynot more than 5.0% by weight, preferably not more than 2.5% by weight.On the other hand, an excessively small amount of alumina makes itdifficult to achieve satisfactory effects according to the presentinvention. Thus, it is required that the amount of alumina is controlledto be at least not smaller than 1.1% by weight.

The zirconia substrate 2 wherein at least the diaphragm portion 10contains a predetermined amount of alumina is obtained by preparing,according to a known manner, a green body of the zirconia substrate 2 inwhich the predetermined amount of alumina is contained at least in aportion of the green body which gives the diaphragm portion (10), andfiring the green body. Described more specifically, the zirconiasubstrate 2 is preferably produced by 1) preparing a zirconia greensheet which gives the base plate 4 and which is formed with an aperture(window 6) by use of a metal mold or by ultrasonic machining ormechanical machining, 2) superposing, on the zirconia green sheet forthe base plate 4, a thin zirconia green sheet which gives the diaphragmplate 8 (diaphragm portion 10) and which contains the predeterminedamount of alumina, and bonding the green sheets together by thermocompression, and 3) firing the green sheets into an integral structure.The zirconia substrate 2 thus obtained exhibits high reliability. Toassure a sufficiently high operating response speed and largedisplacement of the P/E film element, the thickness of the diaphragmportion 10 of the zirconia substrate 2 which carries the P/E unit 18thereon is generally 50 μm or smaller, preferably in a range of 1 μm-30μm, more preferably in a range of 3 μm-15 μm.

Each of the green sheets for the base plate 4 and diaphragm plate 8formed of the suitable zirconia material may consist of a plurality ofthin sheets which are superposed on each other. The diaphragm portion 10of the zirconia substrate 2 may have a convex shape which protrudesoutwards, in a direction away form the window 6, or a concave shapewhich protrudes inwards, toward the bottom of the window 6, in additionto the flat shape as shown in FIG. 1. However, satisfactory effects ofthe present invention can be attained by employing the zirconiasubstrate 2 whose diaphragm portion 10 has the outwardly convex shape(which protrudes upwardly as seen in FIG. 1). While the window 6 of thezirconia substrate 2, in other words, the diaphragm portion 10 has arectangular shape in the present embodiment, the shape of the window 6may be suitably selected from other shapes, such as a circular,polygonal an elliptical shape, and combinations of these shapes,depending upon the application or utility of the P/E film element.Similarly, the number and position of the window 6 may be suitablydetermined.

Next, the P/E unit 18 is formed on the diaphragm portion 10 of thesintered zirconia substrate 2 prepared as described above. Initially, onthe outer surface of the diaphragm portion 10, the lower electrode 12 isformed of a suitable electrode material by a known film-forming method.The electrode material for the lower electrode 12 of the P/E unit 18 isnot particularly limited, provided that the electrode material is anelectrically conductive material which can withstand oxidizingatmospheres having a considerably high temperature. For example, thelower electrode 12 may be formed of a single metal, an alloy of metals,a mixture of a single metal or an alloy and an electrically insulatingceramic such as bismuth oxide, zinc oxide or titanium oxide, or anelectrically conductive ceramic. However, the electrode materialpreferably has a major component consisting of a noble metal having ahigh melting point, such as platinum, palladium or rhodium, or an alloy,such as silver-palladium alloy, silver-platinum alloy orplatinum-palladium alloy. The lower electrode 12 may also be formed ofcermet of platinum, zirconia material for the substrate 2 and/or thepiezoelectric/electrostrictive material for the P/E layer 14. Morepreferably, the lower electrode 12 is made solely of platinum, orinclude as a major component an alloy containing platinum. Where theabove-described cermets are used, the content of the zirconia substratematerial is preferably held in a range of 5-30% by volume, while thecontent of the piezoelectric/electrostrictive material is held in arange of 5-20% by volume.

The lower electrode 12 is formed, using the above-described conductivematerial, by suitably selected one of known film-forming methods whichinclude thick-film forming methods, such as screen printing, spraying,coating and dipping, and thin-film forming methods, such as sputtering,ion-beam method, vacuum vapor deposition, ion plating, CVD and plating.In particular, the thick-film forming methods are favorably employed forforming the lower electrode 12. The lower electrode 12 formed by asuitably selected thick-film forming method is subjected to theconventional heat treatment (firing) for sintering of the lowerelectrode 12, and for integration with the diaphragm portion 10 of thezirconia substrate 2. The thickness of the lower electrode 12 isgenerally not greater than 20 μm, preferably not greater than 5 μm.

On the thus formed lower electrode 12, the P/E layer 14 is formed usinga predetermined piezoelectric/electrostrictive material, by suitablyselected one of the known film-forming methods. The P/E layer 14 may bepreferably formed by one of the above-described thick-film formingmethods, such as screen printing, spraying, coating and dipping. Thethick-film forming method utilizes a paste or slurry which contains as amajor component piezoelectric/electrostrictive ceramic particles havingan average particle size of about 0.01 μm to 7 μm, preferably 0.05 μm to5 μm, so as to form the film-like P/E layer 14 on the outer surface ofthe diaphragm portion 10 of the zirconia substrate 2. In this case, theresultant film element exhibits excellent piezoelectric/electrostrictivecharacteristics. Among the above-described thick-film forming methods,screen printing is particularly favorably employed since it permits finepatterning at a relatively low cost. The thickness of the P/E layer 14is preferably 50 μm or smaller, more preferably in a range of 3 μm to 40μm, to provide a relatively large displacement of the P/E layer 14 witha relatively low voltage.

The piezoelectric/electrostrictive material for forming the P/E layer 14contains magnesia or a component which gives magnesia, in an independentform or in a compound form. The term “a component which gives magnesia”used herein means a component which provides magnesia after the firingof the P/E layer, such as magnesium per se. The piezoelectric/electrostrictive material containing magnesia or the component whichgives magnesia includes as a major component lead zirconate titanate(PZT), lead nickel niobate (PNN), lead manganese niobate, lead antimonystannate, lead zinc niobate, lead titanate or lead nickel tantalate.Alternatively, the piezoelectric/electrostrictive material includes amixture of these materials to which magnesia or magnesium is added. Inother words, the piezoelectric/electrostrictive material includesmagnesia or magnesium independently of thepiezoelectric/electrostrictive composition. Preferably used in thepresent invention is the piezoelectric/electrostrictive material whichcontains magnesia in a compound form within thepiezoelectric/electrostrictive composition which exhibits thepiezoelectric/electrostrictive characteristics. For instance, thepiezoelectric/electrostrictive material preferably contains as a majorcomponent lead magnesium niobate (PMN) or lead magnesium tantalate.Alternatively, the piezoelectric/electrostrictive material preferablyincludes a mixture of these materials and the PZT as described above.

Among the piezoelectric/electrostrictive materials as indicated above,it is recommended to use a material which includes as a major componentone of the following mixtures: a mixture of lead magnesium niobate, leadzirconate and lead titanate; a mixture of lead nickel niobate, leadmagnesium niobate, lead zirconate and lead titanate; a mixture of leadmagnesium niobate, lead nickel tantalate, lead zirconate and leadtitanate; and a mixture of lead magnesium tantalate, lead magnesiumniobate, lead zirconate and lead titanate. Further, a materialcontaining an oxide or other compound of lanthanum, barium, niobium,magnesium, zinc, cerium, cadmium, chromium, cobalt, antimony, iron,yttrium, tantalum, tungsten, nickel, manganese, lithium, strontium, orbismuth may be added to the above-indicatedpiezoelectric/electrostrictive material.

When the piezoelectric/electrostrictive material having three or morecomponents is used, the piezoelectric/electrostrictive characteristicsmay vary depending upon the composition of the components of thematerial. However, a three-component material composed of lead magnesiumniobate, lead zirconate and lead titanate, a four-component materialcomposed of lead magnesium niobate, lead nickel tantalate, leadzirconate and lead titanate, or a four-component material composed oflead magnesium tantalate, lead magnesium niobate, lead zirconate andlead titanate preferably has a composition in the vicinity of phaseboundaries of a pseudo-cubic crystal phase, a tetragonal crystal phaseand a rhombohedral crystal phase. To assure sufficiently highpiezoelectric constant and electromechanical coupling factor, it isparticularly desirable to employ one of the following compositions, thatis, 1) a composition containing 15-50 mol % of lead magnesium niobate,10-45 mol % of lead zirconate and 30-45 mol % of lead titanate, 2) 15-50mol % of lead magnesium niobate, 10-40 mol % of lead nickel tantalate,10-45 mol % of lead zirconate and 30-45 mol % of lead titanate and 3)15-50 mol % of lead magnesium niobate, 10-40 mol % of lead magnesiumtantalate, 10-45 mol % of lead zirconate and 30-45 mol % of leadtitanate.

When the piezoelectric/electrostrictive material includes magnesia orthe magnesia-giving component in a compound form as one constituentelement of the piezoelectric/electrostrictive composition, it is notrequired to adjust the content of magnesia or the magnesia-givingcomponent in the piezoelectric/electrostrictive material. That is,magnesia or the magnesia-giving component is employed in an amountcontained in the piezoelectric/electrostrictive composition of thepiezoelectric/electrostrictive material. When thepiezoelectric/electrostrictive material includes magnesia or themagnesia-giving component in an independent form, the content ofmagnesia or the magnesia-giving component included in thepiezoelectric/electrostrictive material is suitably determined dependingupon the content of alumina (1.1-5.0% parts by weight) which is includedin the diaphragm portion, such that magnesia or the magnesia-givingcomponent reacts with alumina in the diaphragm portion 10 so as toproduce the compound of alumina and magnesia, without adverselyinfluencing the piezoelectric/electrostrictive characteristics of theP/E layer 14.

The P/E layer 14 thus formed on the lower electrode 12 is subjected to asuitable heat treatment (firing) for integration with the lowerelectrode 12 and the zirconia substrate 2, i.e., the diaphragm portion10. The temperature of the heat treatment (firing) of the P/E layer 14for integration with the lower electrode 12 and the diaphragm portion 10is generally controlled to be in a range of 500° C. to 1400° C.,preferably 1000° C. to 1400° C. To avoid changes in the composition ofthe piezoelectric/electrostrictive material of the P/E layer 14 at ahigh temperature, it is desirable to heat-treat or fire the P/E layer 14while controlling the firing atmosphere to include the evaporationsource of the piezoelectric/electrostrictive material. It is alsorecommended to fire the P/E layer 14 while it is covered with a suitablecovering member so that the surface of the P/E layer 14 is not directlyexposed to the firing atmosphere. The covering member may be formed of amaterial similar to that of the zirconia substrate 2.

The P/E layer 14 is sintered into a dense body due to the heat-treatment(firing) as described above, to thereby exhibit the desiredpiezoelectric/electrostrictive characteristics. As a result of theheat-treatment (firing) of the P/E layer 14, magnesia or the componentwhich gives magnesia present in the P/E layer 14 moves toward thediaphragm portion 10 of the zirconia substrate 2 through the lowerelectrode 12, while at the same time, alumina present at least in thediaphragm portion 10 of the zirconia substrate 2 moves toward the uppersurface of the diaphragm portion 10 on the side of the lower electrode12, whereby the alumina component and the magnesia component react witheach other at least on the interface between the lower electrode 12 andthe diaphragm portion 10 located right under the P/E layer 14, so thatthe two components are deposited on the interface in the form ofparticles of a predetermined compound.

The above-described behavior of the alumina component in the diaphragmportion 10 during the heat-treatment of the P/E layer 14 results inreduction in the rigidity of the diaphragm portion 10 of the zirconiasubstrate 2, and the diaphragm portion 10 tends to be easily deformedduring the heat-treatment. This arrangement is effective to avoidoccurrence of the stresses which would be generated due to the firingshrinkage of the P/E layer 14, so that the P/E layer 14 effectivelyexhibits sufficiently high sinterbility or density. Since the P/E layer14 does not suffer from the stresses remaining therein after the firing,the P/E layer 14 is capable of exhibiting its inherent characteristics,to thereby provide the P/E film element having high reliability and highelectromechanical conversion efficiency. The alumina component existingin the diaphragm portion 10 of the zirconia substrate 2 moves toward theupper surface of the diaphragm portion 10 (as seen in FIG. 1), and reactwith the magnesia component which moves toward the diaphragm portion 10from the P/E layer 14 through the lower electrode 12, so as to form thecompound on the interface between the lower electrode 12 of the P/E unit18 and the diaphragm portion 10, which interface is located right underthe P/E layer 14. According to this arrangement, the alumina componentdoes not substantially exist within the diaphragm portion 10. Even inthis case, however, since the rigidity of the diaphragm portion 10 isnot substantially lowered after the heat-treatment of the P/E layer 14,the mechanical strength and the resonance frequency of the film elementdo not reduce.

In the present invention, the zirconia substrate 2 whose diaphragmportion 10 has the outwardly convex shape is preferably employed asdescribed above. The outwardly convex shape of the diaphragm portion 10is generally changed into an inwardly concave shape, protruding into thebottom of the window 6, after the heat-treatment of the P/E layer 14.

On the thus fired P/E layer 14, the upper electrode 16 of the P/E unit18 is formed in the same manner as the lower electrode 12 by suitablyselected one of the known film-forming methods. The upper electrode 12may be preferably formed by screen printing which utilizes a resinateprinting paste or a thick-film printing paste, or a thin-film formingmethod such as sputtering, ion-beam method, vapor deposition, ionplating, CVD and plating. The thickness of the upper electrode 16 isgenerally not greater than 20 μm, preferably not greater than 5 μm. Thetotal thickness of the P/E unit 18 which is the sum of the thickness ofthe upper and lower electrodes 12, 16 and the P/E layer 14 is generallynot greater than 100 μm, preferably not greater than 50 μm.

In the P/E film element produced as described above, the particlesconsisting principally of the compound of alumina and magnesium such asspinel (MgAl₂O₄), are deposited at least on the interface between thediaphragm portion 10 of the zirconia substrate 2 and the lower electrode12 of the P/E unit 18, which interface is located right under the P/Elayer 14. The particles exist on the interface at an occupied area ratioof at least 5% or greater.

Referring to FIG. 2, there is shown a view of an exposed surface of thediaphragm portion 10 of the P/E film element formed as described above,which view is observed by an electron microscope. Described in detail,the P/E unit 18 was peeled off and removed from the zirconia substrate2, and the outer surface of the diaphragm portion 10 located just underthe P/E layer 14 was exposed. As indicated by a black pattern in theview of FIG. 2, the deposited particles 20 consisting principally of thecompound produced by the reaction of alumina and magnesium areuniformly, finely, distributed on the exposed surface of the diaphragmportion 10. The deposited particles 20 exist on the abutting surfaces ofthe diaphragm portion 10 and the lower electrode 12, and provide aneffect of anchoring the lower electrode 12 and the diaphragm portion 10to each other, to thereby effectively enhance the adhesion or bondingstrength therebetween.

The occupied area ratio of the particles 20 of the alumina-magnesiacompound existing on the surface of the diaphragm portion 10 is measuredin the following manner. Initially, the P/E unit 18 (consisting of thelower electrode 12, P/E layer 14 and upper electrode 16) is peeled offand removed from the zirconia substrate 2 by the following method, forinstance. There is first prepared aqua regia consisting of a mixturewhich contains commercially available concentrated hydrochloric acid andconcentrated nitric acid in a volume ratio of 3:1. The P/E film elementis put into the thus prepared aqua regia which is kept at a temperatureof 80° C. The P/E unit 18 is dissolved or decomposed in the aqua regia,so that the P/E unit 18 is removed from the zirconia substrate 2. Theremoval of the P/E unit 18 is easily effected by boiling the aqua resia.Next, the exposed surface of the diaphragm portion 10 of the zirconiasubstrate 2 with the P/E unit 18 being removed is observed by theelectron microscope for obtaining a total area of the particles 20 ofthe alumina-magnesia compound existing on the exposed surface of thediaphragm portion 10, which exposed surface corresponds to the surfacearea of the P/E layer 14 under which the lower electrode 12 was formed.On the basis of the obtained value, the occupied area ratio of theparticles 20 is calculated according to the following formula:

Occupied area ratio (%)=(A/A₀)×100,

wherein

A₀: the surface area of the diaphragm portion 10 which corresponds tothat of the P/E layer 14 under which the lower electrode 12 was formed;and

A: total area of the particles 20 of the alumina-magnesia compound whichexist on the surface area of the diaphragm portion 10 with the P/E unit18 being removed, which surface area corresponds to that of the lowerelectrode 12 right under the P/E layer 14.

The occupied area ratio of the particles 20 consisting principally ofthe alumina-magnesia compound is controlled to be 5% or greater,preferably 10% or greater, more preferably 30% or greater, since theexistence of the particles 20 does not provide sufficient effect if theoccupied area ratio is smaller than the lower limit of 5%. Although theupper limit of the occupied area ratio of the particles 20 is notparticularly limited, it is desirable to control the ratio to be about80% or smaller so as to avoid occurrence of cracking and other defects.

The thus obtained P/E film element according to the present inventiondoes not suffer from reduction in the rigidity of the diaphragm portion,and therefore it is free from reduction in the resonance frequency.Further, the P/E layer of the P/E unit has increased density, wherebythe present P/E film element exhibits excellentpiezoelectric/electrostrictive characteristics with high reliability.While the P/E film element according to the present invention isadvantageously used for various applications such as transducers,sensors and actuators, the present film element is advantageously usedas a piezoelectric/electrostrictive actuator since the elementeffectively undergoes displacement and produces force upon actuation ofthe P/E unit formed on the outer surface of the diaphragm portion. Forexample, the P/E film element of the present invention is used as afilter, various sensors such as an ultrasonic sensor, an angularvelocity sensor, an acceleration sensor and a shock sensor,transformers, microphones, sounding bodies such as a loud speaker,discriminators and various vibrators, oscillators and resonators forpower devices and communication devices. Further, the present filmelement may be particularly advantageously used as a uni-morph, bi-morphor other type of piezoelectric/electrostrictive actuators which producedisplacement in the form of bending or deflection, and are used forservo-displacement elements, pulse-driven motors, ultrasonic motors,piezoelectric fans and others, which elements and motors are describedin “FUNDAMENTALS TO APPLICATIONS OF PIEZOELECTRIC/ELECTROSTRICTIVEACTUATORS”, Kenji Uchino, Japan Industrial Technology Center, publishedby Morikita-shuppan. Further, the present film element is preferablyused as a thick-film condenser element and a display device.

Referring next to FIG. 3 schematically showing another example of apiezoelectric/electrostrictive film element (hereinafter referred to as“P/E film element”) according to the present invention, and to FIG. 4which is an exploded perspective view of the film element of FIG. 3, thefilm element has an integral structure which includes a zirconiasubstrate 22 and a plurality of piezoelectric/electrostrictive units 24(hereinafter referred to as “P/E units”) formed on relevant outersurfaces of thin-walled diaphragm or vibratile portions of the zirconiasubstrate 22. In operation, each of the diaphragm portions of thezirconia substrate 22 is flexed, bent or otherwise deformed uponapplication of a voltage to the corresponding P/E unit 24.

More specifically described, the zirconia substrate 22 has an integrallaminar structure which consists of a relatively thin closure plate(diaphragm plate) 26, a connecting plate (base plate) 28 and a spacerplate (base plate) 30 which is interposed between the closure andconnecting plates 26, 28. These plates 26, 28, 30 are formed of azirconia material. The connecting plate 28 has three communication holes32 which are formed through the thickness of the connecting plate 28with a suitable spacing therebetween. The number, shape, dimensions,position and other parameters of the communication holes 32 may besuitably determined depending upon a specific application or use of theP/E film element. The spacer plate 30 is formed with a plurality ofsquare windows 36 (three in this embodiment). The spacer plate 30 issuperposed on the connecting plate 28 such that the communication holes32 of the connecting plate 28 communicate with the respective windows36. The closure plate 26 is superposed on one major surface of thespacer plate 30 remote from the connecting plate 28, so as to close theopenings of the windows 36 of the spacer plate 30. With the closureplate 26, spacer plate 30 and connecting plate 28 thus superposed on oneanother, three pressure chambers 38 are formed within the zirconiasubstrate 22, such that the pressure chambers 38 communicate with anexterior space through the corresponding communication holes 32.

The zirconia substrate 22 is an integral fired body formed of a suitablezirconia material. While the zirconia substrate 22 of the presentembodiment is a three-layer structure consisting of the closure plate 26(diaphragm plate), spacer plate 30 (base plate) and connecting plate 28(base plate), the substrate may be formed as a four-layer or othermulti-layer integral structure having four or more layers or plates.

Film-like P/E units 24 are formed on the outer surface of the closureplate 26, such that the P/E units 24 are aligned with the respectivepressure chambers 38 as viewed in a plane parallel to the closure plate26. Each of the P/E units 24 includes a lower electrode 40, apiezoelectric/electrostrictive layer 42 (hereinafter referred to as “P/Elayer 42) and an upper electrode 44 which are successively formed by asuitable film-forming method on a portion of the closure plate 26 whichis located in alignment with one of the windows 36 of the zirconiasubstrate 22, that is, on the outer surface of one diaphragm portion ofthe substrate 22. In operation, the pressure in the pressure chamber 38is increased upon actuation of the corresponding P/E unit 24, so that afluid contained in the pressure chamber 38 is effectively dischargedthrough the corresponding communication hole 32. The P/E film elementthus constructed may be used not only as an actuator, but also as asensor or the like which is adapted to generate a voltage signal thatrepresents flexural displacement of the diaphragm portion of thezirconia substrate 22.

In the P/E film element constructed as described above, the particlesconsisting principally of the compound of alumina and magnesia, such asspinel, are deposited at least on the interface between the lowerelectrode (40) and the diaphragm portion (26), which interface islocated just under the P/E layer 42. The particles exist on theinterface in the occupied area ratio of 5% or greater. The particles ofthe alumina-magnesia compound such as spinel may exist within the P/Elayer 42 interposed by and between the upper and lower electrodes 44,40, provided the amount of the particles is relatively small. However,the particles present in the P/E layer 42 are not desirable since suchparticles may adversely influence the piezoelectric/electrostrictivecharacteristics of the film element.

While the P/E film element according to the present invention may beused as actuators, sensors and transducers, particularly advantageouslyas a member of loudspeakers, display devices, servo-displacementelements, pulse-driven motors, ultrasonic motors, acceleration sensors,shock sensors, vibrators, oscillators and resonators, it is to beunderstood that the present film element has other applications known inthe art.

EXAMPLES

To further clarify the present invention, some examples of the P/E filmelement of the invention will be described. However, it is to beunderstood that the invention is not limited to the details of thefollowing examples, but may be embodied with various changes,modifications and improvements which will occur to those skilled in theart, without departing from the principle and scope of the presentinvention defined in the attached claims.

Example 1

As examples of the P/E film element as shown in FIGS. 3 and 4, eightsamples of the P/E film element were prepared by using sintered zirconiasubstrates 22 each having four rectangular windows 36 each of which hada width of 0.3 mm and a length of 0.5 mm. The four windows 36 arearranged in a straight row in the longitudinal direction of the zirconiasubstrate 22, such that the 0.3 mm-long short sides of the windows 36are parallel to the longitudinal direction of the substrate 22 and suchthat the adjacent windows are spaced from each other by a spacingdistance of 0.2 mm. The zirconia substrates 22 of the eight samples ofthe P/E film element included alumina in respective different amounts asindicated in TABLE 1. On the outer surface of each of the diaphragmportions (26) of each of the thus prepared zirconia substrates 22, thelower electrode 40, P/E layer 42 and upper electrode 44 weresuccessively formed in the manner which will be described. Theconnecting plate 28 of the zirconia substrate 22 was formed withcommunication holes 32 each having a diameter of 0.2 mm, such that eachcommunication hole 32 is located at the central portion of thecorresponding window 36 of the spacer plate 30.

In the P/E film element of each sample, the connecting plate 28 andspacer plate 30 (which constitute the base portion of the zirconiasubstrate 22) had a thickness of 150 μm, respectively, when measuredafter firing the substrate 22 while the diaphragm plate 26 giving thediaphragm portion had a thickness of 13 μm when measured after firingthe substrate 22. The connecting plate 28, spacer plate 30 and diaphragmplate 26 were formed of zirconia which was partially stabilized by 3 mol% of yttria. To obtain the zirconia substrate 22 having an integrallaminar structure of the diaphragm plate 26, connecting plate 28 andspacer plate 30, green sheets for these plates 26, 28, 30 are preparedin manners as described below, laminated, bonded under pressure and thenfired.

a) Preparation of a green sheet for the diaphragm plate 26

zirconia powder partially stabilized by 3 mol % (100−x) parts by weightof yttria (average particle size: 0.4 μm) alumina powder (averageparticle size: 0.2 μm) x parts by weight polyvinyl butyral resin(binder) 9.0 parts by weight dioctyl phthalate (plasticizer) 4.5 partsby weight dispersing agent containing sorbitan fatty 2.0 parts by weightacid ester solvent containing 50/50 of toluene and 70 parts by weightisopropyl alcohol

The above-indicated ingredients were blended in a pot mill with zirconiaballs, to provide a slurry having the initial viscosity of 1200 cps(centipoise). The thus obtained slurry was degassed under vacuum, andits viscosity was adjusted to 2000 cps. Then, the slurry was formed by areverse roll coater machine into a green sheet which provides afterfiring the diaphragm portion having a thickness of 8 μm or 13 μm. Thegreen sheet was dried at 50° C. for ten minutes.

b) Preparation of green sheets for the connecting plate 28 and thespacer plate 30

zirconia powder partially stabilized by 3 mol % (100−x) parts by weightof yttria (average particle size: 0.4 μm) alumina powder (averageparticle size: 0.2 μm) x parts by weight polyvinyl butyral resin(binder) 8.0 parts by weight dioctyl phthalate (plasticizer) 4.0 partsby weight dispersing agent containing sorbitan fatty 1.0 parts by weightacid ester solvent containing 50/50 of xylene/(n-) 63 parts by weightbutyl alcohol

The above-indicated ingredients were blended in a pot mill with zirconiaballs, to provide a slurry having the initial viscosity of 2000 cps. Thethus obtained slurry was degassed under vacuum, and its viscosity wasadjusted to 5000 cps. Then, the slurry was formed by doctor blade methodinto green sheets which provide after firing the connecting plate 28 andthe spacer plate 30, respectively, each having a thickness of 150 μm.The green sheets were dried at 80° C. for two hours.

The thus obtained green sheets for the connecting plate 28 and thespacer plate 30 were punched according to respective patterns by meansof suitable metal molds, so as to form the communication holes 32 andthe windows 36, respectively. Then, the green sheet for the diaphragmplate 26 produced as described above was laminated on these greensheets, and bonded thereto by thermo-compression under the pressure of100 kg/cm² at 80° C. for one minute. The thus obtained integral laminarstructure was fired at 1500° C. for two hours. Thus, there were obtainedvarious zirconia substrates 22 which contained respective differentamounts of alumina in the diaphragm portions and the base portionthereof.

On the outer surface of each of the diaphragm portions (26) of thezirconia substrate 22 of each sample, a platinum paste was printed byscreen printing, dried at 120° C. for ten minutes and fired at 1350° C.for two hours, to provide the lower electrode 40 having a thickness of 3μm.

Subsequently, a paste for the P/E layer was printed by screen printingon the lower electrode 40, dried at 120° C. for twenty minutes and firedat 1275° C. for two hours, to provide the P/E layer 42 having athickness of 30 μm. The paste was formed in the following manner. Therewas first prepared a powder of a piezoelectric/electrostrictive materialwhich consisted of 38 mol % of lead magnesium niobate, 24 mol % of leadzirconate and 38 mol % of lead titanate, and in which a part of Pb wassubstituted by Sr and La. The powder had an average particle size of 0.9μm. Then, a composition consisting of 100 parts by weight of the thusprepared powder, 3 parts by weight of acrylic binder and 20 parts byweight of terpineol (solvent) was kneaded to provide the paste for theP/E layer 42 having the viscosity of 100000 cps. The P/E layer 42 wasfired in the furnace in the presence of the powder of thepiezoelectric/electrostrictive material used to form the paste, so as tocontrol the firing atmosphere in the firing furnace.

Upon completion of the firing of the P/E layer 42, a Cr thin film wasformed by sputtering on the P/E layer 42, and a Cu film was formed onthe Cr film to provide the upper electrode 44. Thus, samples Nos. 1-8 ofthe P/E film element as indicated in TABLE 1 were obtained. The obtainedsamples of the film element were subjected to polarization treatment byapplying 40 V between the upper and lower electrodes 44, 40 of each P/Eunit 42 for ten minutes in a forward direction as viewed in thedirection of displacement of the P/E layer 42.

Each of the samples of the P/E film element was evaluated in terms ofthe displacement characteristic and the resonance frequency. Further,the P/E units 24 were removed from the zirconia substrate 22 of eachsample, and the exposed surface of each diaphragm portion (26) wasobserved by the electron microscope. In other words, the exposed surfaceof each diaphragm portion corresponding to the surface area of the P/Elayer 42 was observed, so as to obtain the occupied area ratio of theparticles consisting principally of the compound of alumina and magnesiadeposited on the interface between the lower electrode 40 and thediaphragm portion (26). The results are shown in TABLE 1. Theobservation by the electron microscope (capable of an elemental analysisand an x-ray diffraction analysis) revealed that the deposited particleswere composed of spinel.

To evaluate the displacement characteristic of each sample of the P/Efilm element, a voltage of 30 V was applied between the upper and lowerelectrodes 44, 40 of each P/E unit 24 in the direction of thepolarization treatment as explained above. The amount of displacement ofeach P/E unit was measured by a laser doppler device. The occupied arearatio of the deposited particles of each sample of the film element wasobtained in the following manner. Each sample of the P/E film elementwas put into the above-described aqua regia kept at 80° C., so that theP/E units 24 of each sample were dissolved or decomposed in the aquaresia, and removed from the substrate. The surface of each diaphragmportion (26) from which the corresponding P/E unit 24 was removed wasobserved by the electron microscope capable of an elemental analysis.Then, the total area of the deposited particles present on the exposedsurface of the diaphragm portion was obtained so as to calculate theoccupied area ratio (%) according to the equation as explained above. Inthe following TABLE 1, the displacement characteristic of each sample isrepresented as an average of the amounts of displacement of the four P/Eunits. Similarly, the occupied area ratio of each sample is representedas an average of the occupied area ratios of the four diaphragmportions.

TABLE 1 Content of Thickness of the Displacement Resonance OccupiedSample alumina (x parts diaphragm portion characteristic frequency arearatio No. by weight) (μm) (μm) (MHz) (%) *1  0.2 13 0.14 1.0  2 *2  0.813 0.17 1.0  4 3 1.1 13 0.21 1.0 12 4 1.5 13 0.22 1.0 30 5 2.5 13 0.221.0 80 6 5.0 13 0.19 1.0 95 *7  0.0 13 0.12 1.0  0 *8  0.0  8 0.16 0.7 0 *comparative examples

As is apparent from the results of TABLE 1, the samples Nos. 3-6 of thepresent P/E film element wherein the base portion and the diaphragmportion (26) of the zirconia substrate 22 contained alumina in an amountranging from 1.1-5.0% by weight exhibited excellent displacementcharacteristic. Described more specifically, since thepiezoelectric/electrostrictive material for the P/E layer 42 includedmagnesia in compound form, the magnesia component in the P/E layer 42moved toward the diaphragm portion (26) of the substrate 22 duringfiring of the P/E layer 42, and reacted with the alumina componentincluded in the diaphragm portion (26) on the interface between thediaphragm portion (26) and the lower electrode 40, whereby the particlesof spinel were deposited on the interface. In this case, it wasconfirmed that a part of the alumina component present in the connectingplate 28 and spacer plate 30 moved toward the diaphragm portion (26) andwas involved in the reaction with the magnesia component. The depositedspinel particles existed on the outer surface of the diaphragm portionin relatively high occupied area ratio as indicated in TABLE 1.Accordingly, the samples Nos. 3-6 of the P/E film element according tothe present invention exhibited excellent displacement characteristic asalso shown in TABLE 1. It was also recognized from TABLE 1 that theresonance frequency of each of the samples Nos. 3-6 did not lower sincethe rigidity of each diaphragm portion did not change after the aluminacomponent moved from the inside of the diaphragm portion (26) toward theouter surface of the diaphragm portion (26).

In contrast, in the samples Nos. 1 and 2 of the film element wherein theamount of alumina present at least in the diaphragm portion (26) of thezirconia substrate 22 was insufficient, the spinel particles were notsufficiently deposited on the interface between the diaphragm portion(26) and the lower electrode 40. Further, in the samples Nos. 7 and 8wherein alumina was not included at least in the diaphragm portion (26)of the substrate 22, the spinel particles are not deposited at all.Thus, unlike the P/E film elements according to the present invention,the film elements according to the comparative examples did not exhibitimproved displacement characteristic.

Example 2

As examples of the P/E film element as shown in FIGS. 3 and 4, fivesamples of the P/E film element were prepared by using sintered zirconiasubstrates 22 each having ten rectangular windows 36 each of which had awidth of 0.5 mm and a length of 0.7 mm. The ten windows 36 are arrangedin a straight row in the longitudinal direction of the zirconiasubstrate 22, such that the 0.5 mm-long short sides of the windows 36are parallel to the longitudinal direction of the substrate 22 and suchthat the adjacent windows are spaced from each other by a spacingdistance of 0.2 mm. The diaphragm portions (26) of the zirconiasubstrates 22 of the five samples of the P/E film element includedalumina in respective different amounts as indicated in TABLE 2. On theouter surface of each of the diaphragm portions (26) of each of the thusprepared zirconia substrates 22, the P/E unit 24 was formed in themanner which will be described.

The green sheets which give the respective plates of each zirconiasubstrate 22 were produced in the following manner.

a) Preparation of green sheets for the connecting plate 28 and thespacer plate 30

zirconia powder partially stabilized by 3 mol % of 100 parts by weightyttria (average particle size: 0.8 μm) polyvinyl butyral resin (binder)10 parts by weight dioctyl phthalate (plasticizer) 5 parts by weightdispersing agent containing sorbitan fatty 2 parts by weight acid estersolvent containing 50/50 of toluene and 73 parts by weight isopropylalcohol

The above-indicated ingredients were blended in a pot mill with zirconiaballs, to provide a slurry having the initial viscosity of 1000 cps. Thethus obtained slurry was degassed under vacuum, and its viscosity wasadjusted to 10000 cps. Then, the slurry was formed by doctor blademethod into green sheets which provide after firing the connecting plate28 and the spacer plate 30, respectively, each having a thickness of 150μm. The green sheets were dried at 80° C. for three hours.

b) Preparation of a green sheet for the diaphragm plate 26

zirconia powder partially stabilized by 3 mol % (100−x) parts by weightof yttria (average particle size: 0.8 μm) alumina powder (averageparticle size: 0.2 μm) x parts by weight polyvinyl butyral resin(binder) 9 parts by weight dibutyl phthalate (plasticizer) 4 parts byweight dispersing agent containing sorbitan fatty 2 parts by weight acidester solvent containing 50/50 of toluene and 70 parts by weightisopropyl alcohol

The above-indicated ingredients were blended in a pot mill with zirconiaballs, to provide a slurry having the initial viscosity of 1000 cps. Thethus obtained slurry was degassed under vacuum, and its viscosity wasadjusted to 3000 cps. Then, the slurry was formed by a reverse rollcoater machine into a green sheet which provides after firing thediaphragm portion (26) having a thickness of 9 μm.

The green sheets for the connecting plate 28 and the spacer plate 30prepared as described above were punched according to respectivepatterns by means of suitable metal molds, so as to form thecommunication holes 32 and the windows 36, respectively. Then, the greensheet for the diaphragm plate 26 produced as described above waslaminated on these green sheets, and bonded thereto bythermo-compression under the pressure of 100 kg/cm² at 80° C. for oneminute. The thus obtained integral laminar structure was fired at 1500°C. for two hours. Thus, there were obtained various zirconia substrates22 whose diaphragm portions (26) contained the respective differentamounts of alumina as indicated in TABLE 2. The zirconia substrates 22in the samples Nos. 9-12 of the P/E film element had the diaphragmportions (26) each having the outwardly convex shape (whose amount ofprotrusion is about 20 μm.)

On the outer surface of each of the diaphragm portions (26) of theceramic substrate 22 of each sample, a platinum paste was printed byscreen printing, dried at 120° C. for ten minutes and fired at 1350° C.for two hours, to provide the lower electrode 40 having a thickness of 5μm.

Subsequently, the P/E layer 42 was formed on each of the thus formedlower electrode 40 of each sample as described below. Initially, a pastefor the P/E layer was prepared by using a powder of apiezoelectric/electrostrictive material which consisted of 38 mol % oflead magnesium niobate, 24 mol % of lead zirconate and 38 mol % of leadtitanate, and in which a part of Pb was substituted by Sr and La. Thispaste for the P/E layer 42 was printed by screen printing on the lowerelectrode 40, dried at 120° C. for twenty minutes and fired at 1275° C.for two hours, to provide the P/E layer 42 having a thickness of 30 μm.The P/E layer 42 was fired in the furnace in the presence of the powderof the piezoelectric/electrostrictive material used to form the paste,so as to control the firing atmosphere in the firing furnace.

Upon completion of the firing of the P/E layer 42, a Cr thin film wasformed by sputtering on the P/E layer 42, and a Cu film was formed onthe Cr film to provide the upper electrode 44. Thus, samples Nos. 9-13of the P/E film element as indicated in TABLE 2 were obtained. Theobtained samples of the film element were subjected to polarizationtreatment by applying 100 V between the upper and lower electrodes 44,40 of each P/E unit 24.

Each of the thus obtained P/E film elements was evaluated in terms ofthe displacement characteristic, the resonance frequency and theoccupied area ratio of the spinel particles deposited on the interfacebetween the diaphragm portion (26) and the lower electrode (40), in thesame manner as in the EXAMPLE 1. The results are shown in TABLE 2. Inthe following TABLE 2, the displacement characteristic of each sample isrepresented as an average of the amounts of displacement of the ten P/Eunits. Similarly, the occupied area ratio of each sample is representedas an average of the occupied area ratios of the ten diaphragm portions.

TABLE 2 Content of alumina in the diaphragm Thickness of theDisplacement Resonance Sample portion diaphragm portion characteristicfrequency Occupied area No. (x parts by weight) (μm) (μm) (MHz) ratio(%) *9 0.8 9 0.23 0.6  3 10 1.1 9 0.37 0.6 10 11 1.5 9 0.36 0.6 25 122.5 9 0.37 0.6 65 *13  0.0 9 0.19 0.6  0 *comparative examples

As is apparent from the results of TABLE 2, the samples Nos. 10-12 ofthe P/E film element of the present invention wherein the diaphragmportions (26) included alumina in respective suitable amounts asspecified according to the present invention exhibited excellentdisplacement characteristic.

In the P/E film element constructed according to the present invention,the magnesia component included in the piezoelectric/electrostrictivematerial for the P/E layer and the alumina component included at leastin the diaphragm portion of the zirconia substrate move toward at leastthe interface between the diaphragm portion and the lower electrode,which interface is located right under the P/E layer. The aluminacomponent and the magnesia component react with each other at theinterface, to thereby deposit the particles of the compound consistingof the two components. According to this arrangement, the rigidity ofthe diaphragm portion is effectively reduced during the firing of theP/E layer and the diaphragm portion is likely to be easily deformed, sothat the P/E layer is effectively sintered into a sufficiently densebody. Further, the present P/E film element is free from theconventionally experienced residual stresses due to the firing shrinkageof the P/E layer, assuring high operating reliability and highelectromechanical conversion efficiency.

In the present P/E film element constructed as described above, thealumina component existing in the diaphragm portion moves toward thesurface of the diaphragm portion during the firing of the P/E layer, andthe amount of alumina within the diaphragm portion is considerablyreduced. However, the rigidity of the diaphragm portion does notsubstantially lower after the firing, so that the film element does notsuffer from reduction in the resonance frequency.

In the P/E film element according to the present invention, theparticles of the compound consisting of alumina and magnesia deposit andexist at least on the interface between the diaphragm portion and thelower electrode right under the P/E layer. The deposited particlesprovide anchoring effect for bonding the diaphragm portion and the lowerelectrode to each other, assuring enhanced bonding strength or adhesiontherebetween.

What is claimed is:
 1. A piezoelectric/electrostrictive film elementcomprising: a ziroconia substrate having at least one window, and adiaphragm portion formed as an integral part of said ziroconia substrateand closing each of said at least one window and where said diaphragmportion contains an alumina component in an amount of 1.1-5.0 parts byweight before firing; and a piezoelectric/electrostrictive unit filmincluding a lower electrode, a piezoelectric/electrostrictive layer andan upper electrode, which are formed in the order of description on anouter surface of said diaphragm portion by a film-forming process, saidpiezoelectric/electrostrictive layer being formed using apiezoelectric/electrostrictive material which contains magnesia or acomponent which gives magnesia in an independent form or in a compoundform; wherein said diaphragm portion and said lower electrode define aninterface directly under said piezoelectric/electrostrictive layer, andparticles consisting essentially of a compound of alumina and magnesiaexist at least on said interface at an occupied area ratio of 10% orgreater, wherein occupied area ratio is defined by (%)=(A/A₀)×100, A₀being the surface area of the diaphragm portion which corresponds tothat of the piezoelectric/electrostrictive layer under which the lowerelectrode was formed, and A being the total area of particles consistingessentially of the alumina-magnesia compound which exist on the surfacearea of the diaphragm portion with the piezoelectric/electrostrictiveunit being removed, which surface area corresponds to that of the lowerelectrode right under the layer, to thereby relax said diaphragm portionduring sintering of said piezoelectric/electrostrictive layer tofacilitate sintering of said piezoelectric/electrostrictive layer.
 2. Apiezoelectric/electrostrictive film element according to claim 1,wherein said particles exist on said interface at said occupied arearatio of 30% or greater.
 3. A piezoelectric/electrostrictive filmelement according to claim 1, wherein said compound of alumina andmagnesia is spinel.